netdev - Re: [Query]: DSA Understanding

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Message-ID: <20180809172334.GA25449@lunn.ch>
Date:   Thu, 9 Aug 2018 19:23:34 +0200
From:   Andrew Lunn <andrew@...n.ch>
To:     "Lad, Prabhakar" <prabhakar.csengg@...il.com>
Cc:     netdev <netdev@...r.kernel.org>
Subject: Re: [Query]: DSA Understanding

> Its coming from the switch lan4 I have attached the png, where
> C4:F3:12:08:FE:7F is
> the mac of lan4, which is broadcast to ff:ff:ff:ff:ff:ff, which is
> causing rx counter on
> PC to go up.

So, big packets are making it from the switch to the PC. But the small
ARP packets are not.

This is what Florian was suggesting.

ARP packets are smaller than 64 bytes, which is the minimum packet
size for Ethernet. Any packets smaller than 64 bytes are called runt
packets. They have to be padded upto 64 bytes in order to make them
valid. Otherwise the destination, or any switch along the path, might
throw them away.

What could be happening is that the CSPW driver or hardware is padding
the packet to 64 bytes. But that packet has a DSA header in it. The
switch removes the header, recalculate the checksum and sends the
packet. It is now either 4 or 8 bytes smaller, depending on what DSA
header was used. It then becomes a runt packet.

Florian had to fix this problem recently.

http://patchwork.ozlabs.org/patch/836534/

You probably need something similar for the cpsw.

    Andrew